1. Field of the Invention
The present invention relates in general to diagnosis of failing integrated circuits, and more specifically to a radiation induced fault analysis system and method for determining location of defects on failed integrated circuits.
2. Description of the Related Art
A variety of radiation-based stimulation circuit testing techniques are known for failure or performance analysis of semiconductor devices. Radiation stimulation involves the use of various forms of radiation or emissions or the like with sufficient energy to modify operating behavior of semiconductor circuitry for the purpose of identifying potential problem areas of the semiconductor device. Although many types of radiation may be used, it is desired that the radiation convey sufficient energy to modify circuit operation for purposes of testing the limits of circuit operation. A laser beam, for example, is capable of conveying a significant level of power without damaging semiconductor circuitry and thus is often the radiation of choice for testing. The circuit modification may be any one or more of multiple types, such as modified timing of a device (e.g., transistor, gate, node, etc.), modified voltage level, modified current level, etc. A timing adjustment may reveal, for example, a race condition between two or more circuit paths thereby limiting maximum frequency of operation of the semiconductor device. Similarly, a marginal voltage or current level affecting pass-fail behavior may be revealed using radiation perturbation during testing.
Laser assisted device alteration (LADA) is a known laser scan technique used in the failure analysis of semiconductor devices. A laser generated by a laser scanning microscope (LSM) or the like is used to alter the operating characteristics of transistors, metal interconnects or other components on the semiconductor device under test (DUT) while it is electrically stimulated. Certain operating characteristics of the laser (e.g., wavelength, size, power, etc.) may be selected or otherwise adjusted to modify circuit characteristics, such as circuit timing and/or voltage or current levels. For example, a laser operating at a wavelength of approximately 1,064 nanometers (nm) produces localized photocurrents within active transistor layers in which the photo-generated currents modify circuit timing or voltage levels. Alternatively, a laser operating at a wavelength of approximately 1,340 nm produces localized heating which also alters circuit timing (e.g., slowing down of logic transitions). It has been observed that photocurrent injection enables significantly larger timing shifts as compared to thermally induced alteration, so that photocurrent injection is more commonly used for “standard” LADA testing. Electrical stimulation of the DUT is usually performed by automated test equipment (ATE) which applies an ATE test loop or test pattern to the DUT and monitors the results. The term “ATE” as used herein refers to any test equipment or electronic device or system or the like which provides electrical stimulation to a DUT and which monitors results. The test pattern is designed by test engineers with multiple test vectors applied in sequential order to perform critical timing testing. The test equipment may also adjust one or more test variables, such as laser power, supply voltages, temperature, clock frequency, etc., to adjust operation relative to a pass-fail boundary of the voltage-frequency relationship (which may be plotted on a “shmoo” graph to illustrate the pass-fail boundary as known to those skilled in the art).
Techniques, such as laser assisted device alteration (LADA) or soft defect localization (SDL), use a laser to analyze voltage, frequency and/or temperature sensitive devices to identify internal failing circuit locations caused by soft defects on the device. A “soft” defect is one in which circuit behavior depends upon voltage, frequency and/or temperature variations. Whereas the LADA and SDL techniques, among others, are useful for identifying soft defects, they are not effective for determining “hard” defect locations. A “hard” defect is usually caused by a physical defect or the like during manufacture which modifies circuit behavior. The modified circuit behavior caused by a hard defect is generally independent of voltage, frequency and/or temperature variations. Thus, a device under test with a hard defect usually produces a failed test result regardless of changes to any variable in the test system so that LADA techniques are not feasible. Software defect localization techniques implement design models, fault models, and simulations in an attempt to predict potential defect locations inside failing die based on combinations of failing test results. Software techniques, however, are based on simplified circuit and test models which may or may not accurately duplicate the fault(s). Also, software techniques are typically only effective for scan based test patterns and do not work for functional tests. This is partly due to the added complexity of functional patterns and finite computer computational resources available.